Mass spectrometry is a powerful analytical tool used in the identification of unknown compounds and elements. The technique measures the mass-to-charge ratio (m/z) of individual ionized molecules. Because the charge of those ions is usually known (and is usually plus or minus z, the charge of one electron), the mass of the ion may be deduced, and the molecule may be identified.
The functional units of a modem mass spectrometer 100 are shown in FIG. 1. An inlet 110 provides a location for introduction of a sample. The inlet may be configured to accept a solid, a liquid or a gas. The inlet is equipped to permit introduction of the sample into a vacuum chamber 150 that is kept evacuated using vacuum pumps 118.
The sample passes into an ion source 112 that provides the charged molecular particles that are to be tested. In the case of a gaseous sample, ions are generated by bombarding the sample molecules with a beam of energetic electrons. Solid and liquids may first be vaporized by evaporation or sublimation, then subject to electron ionization. Less energetic ionization techniques have been developed based on chemical or desorption ionization. Those techniques are used where it is desirable to preserve a molecular structure of the sample.
The ions produced by the ion source 112 are directed into an analyzer 114 where the ions are sorted according to their mass-to-charge ratio. The sorted ions are then processed by a detector 116, where ion flux is converted into a proportional electrical current. That electrical current is recorded and analyzed by a data processing system 120 that contains analysis algorithms for producing mass spectra data outputs 122.
Several techniques have evolved for sorting and detecting ions in a modern mass spectrometer. The most widely used of those techniques include magnetic sector alyzers, quadrupole mass filters, quadrupole ion traps, Fourier transform ion cyclotron resonance spectrometers and time-of-flight mass analyzers. Of those techniques, a well-designed Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) provides extremely high resolution for relatively low cost. While the present invention is applicable to any of the above-mentioned spectrometer configurations, it will be described herein with reference to the FT-ICR MS.
Most mass spectrometers operate by altering the trajectory of an ion in a magnetic field. The new trajectory depends on properties related to the mass-to-charge ratio of the ion; i.e., at a given velocity, the trajectory of a more massive ion is altered less than that of a less massive ion. By subjecting ions of differing m/z to a fixed magnetic field, those ions may be separated for detection and analysis.
An analyzer/detector 200 of a typical FT-ICR MS, shown in partial section in FIG. 2, includes a group of plates 211-216 surrounding a roughly cubic cell 225. Ions entering the cell 225 along the path 250 are trapped electrostatically in a homogeneous magnetic field 220. The ions orbit around the field lines in orbits 230. Trapping plates 215, 216 keep the ions from spreading along the field lines.
The orbital motion 230 is induced by applying an RF pulse to the excitation plates 213, 214. Ions having a particular mass are excited by a particular resonant frequency, with less massive ions having a higher resonant frequency and more massive ions having a lower resonant frequency. In practice, a short RF pulse containing a specific group or range of frequencies is applied across the excitation plates 213, 214 to move all the ions sequentially.
As the ions are excited by their respective resonant frequencies, the orbit 230 of those ions enlarges, bringing them in proximity with the detection plates 211, 212. The ions induce a faint signal across the plates, which is received and analyzed by data system 120 (FIG. 1). Alternatively, the orbit of selected ions can be enlarged to a diameter that exceeds the dimensions of the cell by increasing the amplitude of the signal at the resonant frequency. This process, known as ejection, brings the selected ions outside the cell where they are eliminated by the vacuum pump. The ejection process is used to eliminate ions of chosen m/z to improve the detection limit of other ions, particularly when the concentration of the ejected ions is large compared to that of the ions of interest. For example, ejection may be used in detecting gasses in trace amounts with hydrogen.
The electronics 300 of an FT-ICR MS, shown as a simplified schematic in FIG. 3, includes detection electronics 310, excitation electronics 350 and a computer control 380. The excitation electronics 350 includes a digital signal processor (DSP) 352 controlling a digital to analog converter (DAC) 354. The output from the DAC 354 is filtered by filter 356 to remove unwanted signal components. The DSP 352, DAC 354 and filter 356 are used in existing FT-ICR MS equipment for the excitation and ejection of particles having a moderate to large molecular mass. The DAC 354 is controlled to generate a waveform containing the resonant frequencies used to excite or eject components that have masses corresponding to those resonant frequencies. That waveform is applied across the excitation plates 213, 214 of the FT-ICR MS cell 200.
Detection electronics 310 receive the resulting signal from the detection plates 211, 212. An analog-to-digital converter (ADC) 317 converts the analog signal to a digital signal, which is processed by the DSP 315. Both the excitation electronics 350 and the detection electronics 310 are controlled by a computer control 380. The computer control 380 executes instructions for initiating and operating the excitation and detection electronics 350, 310, as well as other components, and stores data for running particular routines involving those components. The computer control also stores and performs analysis of data received from the detection electronics 310.
In cases where low-mass elements, such as hydrogen and helium, must be analyzed, the required excitation frequencies are too high to be generated using the DAC 354 as used in the known art. That is because, under Nyquist sampling theory, the conversion rate of the DAC must be at least double the desired sampled output frequency to avoid aliasing. Prior art excitation electronics are designed to avoid aliasing; i.e., they are capable of sampling at a rate twice the highest specified output frequency. Commercially available DACs do not typically have a conversion rate sufficient to provide excitation signals having the resonant frequencies of light elements in a typical FT-ICR MS cell.
To circumvent the limited conversion rate of the DAC, it is known to switch control of the excitation plates 213, 214 using a selector 359 to a separate signal source such as a configurable oscillator 358. Such a configurable oscillator, when commercially available, is expensive and often does not have the resolution of the DSP/DAC components 352, 354 used in the analysis of higher-mass particles.
There is therefore presently a need to provide a method and apparatus for detecting low-mass particles using mass spectrometry. Particularly, there is a heed for a technique for generating an excitation frequency that is high enough to excite low-mass ions such as helium and hydrogen. The technique should preferably be implemented with a minimum of additional cost and should have high resolution. To the inventor's knowledge, there is currently no such technique available.